The present invention relates to a method of accurately metering plastic over the cavity of an open compression mold, advantageously for large products made from plastics, such as polyesters, polyolefins, etc., plain or reinforced with glass, etc. It is particularly suitable for depositing such plastics on top of decorative materials, which may be plastic or other materials or combinations of materials. Such a system is shown in copending U.S. patent application Ser. No. 09/130,864, the disclosure of which is incorporated herein by reference.
Large components used often in large quantities, such as in the automotive and appliance industry, can be produced both by injection molding and by compression molding. A problem in injection molding is that the molds are often quite complicated, especially due to the need to distribute the injected hot plastic over large areas, and in relatively heavy and often unequal thicknesses. Also, the injection process makes it difficult to decorate the product during the molding process with a special finish, as for added appearance and sales appeal, and injection requires the added cost of thermoforming for deep draw parts and in general is more difficult than the extrusion/compression process.
In order to overcome these disadvantages, the almost century old compression molding technique, much ignored during the last 50 years, has resurfaced and is now often successfully used for molding large, usually decorated products. The typical compression molding is done in vertical presses. Thermosetting molding material is placed into the heated cavities either in the cold or preheated state and then cured in the closed, hot mold. In a limited way, decoration and reinforcements, with other layers, are possible with this method.
However, thermosetting materials are not as suitable, for better quality, for a wider range of applications, and economically, as thermoplastics could be. The problem now is to supply hot plastic in sufficient quantity, evenly distributed, into the cavity or over an inserted decorative sheet. it is important that little time is wasted between the application of the plastic and the closing of the press. Some methods have used extruded, hot or reheated, very thick film (possibly 5-10 mm), cut into size and shape and then place it over (or into) the cavity. Another attempt is to move the extruder right over the cavity and extrude a heavy sheet right over the cavity while slowly retracting the extruder and extruding die head. The problem here is to coordinate the motion of the extruder so that the proper amount of plastic is dispensed. This is dependent on the speed of motion and the viscosity of the plastic.
Another problem with this method is that a conventional extruder (especially for this application requiring often several kilograms per shot) must be very large, weighing several tons, and is difficult to move accurately; by necessity, such motion is very slow. A serious difficulty is to guide the die head over the cavity, properly aligned, while moving the heavy extruder back and forth. While it is not too difficult to maintain alignment in the vertical plane with the center line of the press, the heavy extruder die is not held from vibrating up and down. On a large product of for example 1.5 meters width, the extruder die head could weigh one ton or more, and this would be at the end of an extruder barrel of 3 meters or more in length. Horizontal alignment would be very difficult to achieve.
Also, the extruder delivers only intermittently, since the time to deliver takes maybe 10 seconds while the cooling process in the mold may take 2 minutes or more. To run a conventional extruder intermittently to this extent is not desirable because it makes it difficult to control and achieve a uniform viscosity of the plastic, and an evenly thick plastic sheet.
The shape (contour) of the product may not be square, and it would be practically impossible to vary the width of the extruded sheet, thus creating a large amount of wasted plastic outside of the actual shape of the cavity.
The sheet thickness could possibly be varied over certain areas if required, using a specially designed and very expensive sectioned die head, in combination with a modulated speed of retracting the die head while delivering the plastic into the mold. Such a method would require one to control and correlate all the critical conditions of speed, temperature, extruder output and extruder die opening, in order to achieve different plastic wall thicknesses in the product. While this would be possible, it would not be very practical.
Another problem is the necessarily slow horizontal motion of the extruder as it dispenses the plastic while retracting; this may take from 5-10 seconds. Added to this is the time it takes to complete the withdrawal of the die head from the molding area so that the clamp can close to start the molding process, which may, due to the heavy mass of the extruder, take another 5-10 seconds, so that from the time the depositing begins it may be 10-20 seconds before the mold can close and start to shape the plastic. During this time, the temperature (and viscosity) of the extruded sheet may be substantially different from one end to the other.
It is, therefore, an object of this invention to limit the moving masses of machinery, and to insure that the plastic is deposited only where required, at an adjustable temperature and viscosity and at an adjustable volume suitable for the desired product. By reducing the moving masses, the time for depositing the plastic will be greatly reduced, and the delivery system out of the way of the closing mold in a much shorter time than before.
Hot, plastic globs will weld together to form a homogeneous mass as the molding pressure is applied under the fast closing mold. Any film used as an insert will bond to the plastic while under pressure.
The present invention prepares a molded article by the process which comprises: providing a mold cavity and a mold core adjacent said mold cavity, said mold core and mold cavity forming a molding area therebetween; providing a hot plastic delivery means positioned adjacent said molding area and delivering hot plastic from said hot plastic delivery means to at least one shooting pot which maintains said plastic at an elevated temperature for delivery to said molding area; and delivering hot plastic to said molding area from said at least one shooting pot and forming said hot plastic in said mold cavity into a molded article having the shape of said mold cavity at least in part by said mold core which forms the plastic into the shape of the mold cavity.
The apparatus of the present invention comprises: a mold cavity; a hot plastic delivery means positioned adjacent said mold cavity; a mold core adjacent said mold cavity, said mold core and mold cavity forming a molding area therebetween; at least one shooting pot connectable to said hot plastic delivery means for receiving hot plastic from said delivery means, wherein said shooting pot maintains said plastic at an elevated temperature for delivery to said mold cavity; means for delivery of hot plastic from said shooting pot to said molding area; and means for forming said hot plastic in said mold cavity into a molded article having the shape of said mold cavity including said mold core which forms the plastic into the shape of the mold cavity.